1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive comprising asynchronous/synchronous gain control for fault tolerant detection of servo sync mark after a head switch.
2. Description of the Prior Art
Disk drives may comprise multiple disks with corresponding heads for accessing a top and bottom surface of each disk. Typically only one head is active at a time for reading data from or writing data to a particular disk surface. FIG. 1 illustrates a prior art format for a disk surface 2 comprising a plurality of radially spaced, concentric tracks 4. Each track 4 comprises a plurality of data sectors and a plurality of embedded servo sectors 6. Each servo sector 6 comprises a preamble 8 for synchronizing gain control and timing recovery, a sync mark 10 for synchronizing to a data field 12 comprising coarse head positioning information such as a track number, and product servo bursts 14 which provide fine head positioning information.
When the disk drive switches the active head, it must synchronize to the embedded servo sectors 6 of the newly selected disk surface before it can perform a read or write operation. The disk drive can synchronize to the embedded servo sectors 6 once the servo sync mark 10 in one of the embedded servo sectors 6 is detected. If the embedded servo sectors 6 and heads are aligned perfectly with respect to one another, then the location of the servo sectors 6 on the newly selected disk surface after a head switch would be known relative to the previously selected disk surface. The disk drive opens a detection window where the servo sync mark 10 should occur, wherein the detection window helps prevent miss-detecting a false sync mark. However, because small misalignments occur between the disks as well as the heads, the detection window may be opened too late and the servo sync mark 10 missed. It may take several servo sectors before the servo sync mark 10 is detected.
There is, therefore, a need to improve the reliability of servo sync mark detection after a head switch to ensure fast, fault tolerant synchronization to the embedded servo sectors.